(1) Field of the Invention
The invention relates to genetically engineered modified vaccinia Ankara (MVA) or recombinant MVA (rMVA) vaccines with improved stability during extended passage. Specifically, the invention relates to genetically stable rMVA vaccines expressing cytomegalovirus (CMV) antigens such as an IEfusion protein. The invention also relates to methods for improving genetic stability and maintaining immunogenicity of rMVA vaccines after serial passage. The invention further relates to methods for the preparation of the rMVA vaccines.
(2) Description of the Related Art
Modified vaccinia Ankara (MVA) is a genetically engineered, highly attenuated strain of vaccinia virus that does not propagate in most mammalian cells (Daftarian et al. 2005)). This property minimally impacts viral or foreign gene expression because the ability of MVA to replicate in mammalian cells is blocked at late stage viral assembly. MVA also has a large foreign gene capacity and multiple integration sites, two features that make it a desirable vector for expressing vaccine antigens. MVA has a well-established safety record and versatility for the production of heterologous proteins (Drexler et al. 2004; Ramirez et al. 2000; Stickl et al. 1974; Stittelaar et al. 2001; Werner et al. 1980). In fact, MVA-based vaccines for treatment of infectious disease and cancer have been developed and reached Phase I/II clinical trials (Acres 2007; Cosma et al. 2003; Gilbert et al. 2006; Peters et al. 2007; Rochlitz et al. 2003).
MVA has an extensive history of successful delivery into rodents, Rhesus macaques, and other non-human primates, and more recently as a clinical vaccine in cancer patients (Gilbert et al. 2006; Peters et al. 2007; Rochlitz et al. 2003). MVA is avirulent because of the loss of two important host-range genes among 25 mutations and deletions that occurred during its repeated serial passage in chicken cells (Antoine et al. 1998; Meyer et al. 1991). In contrast to NYVAC (attenuated Copenhagen strain) and ALVAC (host-range restricted Avipox), both early and late transcription are unimpaired making MVA a stronger vaccine candidate (Blanchard et al. 1998; Carroll et al. 1997a; Carroll et al. 1997b; Zhang et al. 2007). Studies in rodents and macaques affirm the safety of MVA, including protection against more virulent forms of pox viruses in challenge models and lack of persistence three months beyond initial dosing administration (deWaal et al. 2004; Earl et al. 2007; Hanke et al. 2005). Similarly, a therapeutic vaccination with MVA expressing HIV-nef demonstrated its safety in HIV-infected individuals (Cosma et al. 2003). Finally, MVA immunizations of malaria patients coinfected with HIV and/or TB confirm the safety of the vector and its ability to partially protect against a heterologous malaria strain (Gilbert et al. 2006; Moorthy et al. 2003).
These properties make MVA appealing as a vaccine vector for CMV antigens in individuals who are both severely immunosuppressed and experiencing additional complications such as malignancy or organ failure, thereby requiring a transplant. CMV infection is an important complication of transplantation procedures and affects a wide variety of individuals including newborns and HIV patients with advanced disease (Pass et al. 2006; Sinclair et al. 2006; Zaia 2002). Individuals who are previously CMV-infected or receiving a CMV-infected solid organ or stem cell allograft are candidates for a vaccine strategy that targets the cellular reservoir of the virus (Zaia et al. 2001).
Several antigens have been identified as being associated with protective immunity against CMV in transplant recipients. These include the tegument protein pp65 (UL83) and the immediate-early 1 (IE1 or UL123) global gene expression regulator (Boeckh et al. 2006; Cobbold et al. 2005; Cwynarski et al. 2001; Einsele et al. 2002; Gratama et al. 2001). In addition, a recent proteomic study of the whole CMV genome divided into overlapping peptides showed that pp65 stimulates substantial levels of both CD8+ and CD4+ T cells, while IE1 mainly stimulates CD8+ T cells, and the related IE regulator referred to as IE2 (UL122) stimulates a vigorous CD8+ and a smaller CD4+ T cell memory response by a large percentage of asymptomatic CMV-positive adults (Sylwester et al. 2005). Other antigens are also recognized with robust cellular immune responses, but the evidence for these three antigens to be highly recognized in a majority of CMV-infected healthy subjects and transplant patients (Gallez-Hawkins et al. 2005) is compelling and justifies their inclusion into a vaccine to prevent viremia and control infection.
Because MVA only replicates in the cytoplasm of cells with its own vaccinia transcriptional system (which does not recognize other viral and cellular promoters), vaccinia viral promoters are used to direct foreign antigen gene expression efficiently in recombinant MVA (rMVA) systems. There are two types of vaccinia promoters widely used to direct foreign gene expression in recombinant MVA. One is pSyn, which contains both vaccinia early and late promoter sequences optimized for high level protein expression (Chakrabarti et al. 1997). The other is modified H5 promoter (mH5), which contains both native early and late vaccinia promoter regions. pSyn has stronger overall promoter activity than mH5, but the early activity of the mH5 promoter is three- to five-fold stronger than the pSyn series.
It has been reported that in vitro expression levels of foreign antigens by an rMVA vaccine are correlated with the rMVA vaccine's immunogenicity (Wyatt et al. 2008b). For example, mice immunized with the rMVAs expressing high level of human immunodeficiency virus (HIV) Env protein had about 15-fold higher titers of Env antibodies and several fold higher frequencies of Env-specific CD8+ and CD4+ T cells than mice immunized with rMVAs expressing low level of Env (84). However, after serial passage, the foreign antigen expression may be reduced and rendered unstable, which can result in diminished immunogenicity.
Thus, while MVA is an attractive viral vector for recombinant vaccine development, genetic instability and diminished immunogenicity are significant concerns after serial passage. The beneficial effect of high antigen expression levels and improved immunogenicity can be limited by the tendency of rMVA to delete genes unnecessary for its lifecycle. Previous reports suggest that instability of rMVA vaccines may be related to toxicity of foreign protein in the gene region in which it is inserted or the promoter that controls foreign protein expression (Timm et al. 2006; Wyatt et al. 2008a). For example, rMVA viruses expressing HIV Env protein and other rMVAs were found to have non-expressing mutant virus accumulation after serial passage (Wyatt et al. 2008a). rMVA expressing hemagglutinin-neuraminidase (HN) glycoproteins under control of pSyn was previously reported to replicate poorly (Wyatt et al. 1996). The non-expressing mutants and poor replications of rMVAs were reported to be likely due to toxicity of the expression of foreign proteins (Wyatt et al. 2008a; Wyatt et al. 1996). However, an rMVA expressing a mutated truncation of Env is found to have enhanced genetic stability and immunogenicity relative to rMVAs expressing a full-length Env (Wyatt et al. 2008a). Thus, a higher expression level of foreign antigens driven by a strong promoter in rMVA vaccines does not always result in higher immunogenicity after serial passage. Genetic instability and diminished immunogenicity after serial passage have not been fully understood.
It will be advantageous to develop an rMVA vaccine with stable expression of foreign protein antigens and immunogenicity after serial passage, which will enable the use of MVA as a clinical vector for a broader portfolio of infectious pathogens and cancer antigens.